A MIOCENE PULL-APART DEFORMATION ZONE AT THE WESTERN MARGIN OF THE JAPAN SEA BACK-ARC BASIN: IMPLICATIONS FOR THE BACK-ARC OPENING MODE

Abstract

Opening mode of the Japan Sea is studied from the viewpoint of geologic structure around the western marginal fault zone of the back-arc basin. Latest reflection seismic survey in the Fukue Basin, located at the western end of the rifted sliver of southwest Japan, delineates an NNE-SSW transcurrent fault that is an extension of the Yangsan Fault running along the eastern coast of the Korean Peninsula. As suggested by paleomagnetic studies in Korea, the Yangsan Fault was activated with dextral motions during the early Miocene rifting event of the Japan Sea. Our offshore seismic study indicates that the NNE-SSW fault zone was simultaneously active with dextral sense because it constitutes a pull-apart depression on a right stepping of the fault. The Fukue Basin was under continuous transtensional stress regime during the Japan Sea rifting accompanied with a clockwise rotation of southwest Japan. It is discordant from the previous tectonic model of two-stage (parallel and fan-shaped) back-arc opening, which requires compressive regime with sinistral fault motions to the west of rotation pivot of southwest Japan in the second stage. The southward transportation of the rifted sliver with formation of pull-apart basins was the dominant mode along the western margin of the Japan Sea throughout the opening event. The southern part of the marginal fault zone has been immune from post-opening tectonic events, such as the late Miocene strong inversion on the back-arc margin of southwest Japan or the Quaternary E-W tectonic stress in the Japan Sea reactivating the northern sector of the fault zone, Yangsan Fault.